The Royal Air Force has declared its third and final Airseeker signals intelligence aircraft fully operational ahead of schedule.

The reconnaissance plane, known more formally as the RC-135W Rivet Joint, will be joined by a ground-based analysis unit.

It is the last of three Airseekers to come into service, with the aircraft intended to replace the Nimrod R1, which was retired in 2011.

Not to be confused with the better-known Nimrod maritime patrol aircraft, the R1 was an electronic intelligence gathering (ELINT) variant.

Acquisition of the three new planes, which are expected to serve until 2035, was budgeted at £634m.

The two already in service have helped support efforts against Daesh in the Middle East while working alongside Sentry, Shadow and Sentinel aircraft, as well as Reaper drones and RAF fighter jets.

The Chief of the Air Staff, Air Chief Marshal Sir Stephen Hillier, said: "A vital part of our ISTAR [Intelligence, Surveillance, Target Acquisition and Reconnaissance] Force, it [Airseeker] provides critical information and understanding to decision-makers in an increasingly complex, congested and contested battlespace.

"It offers essential support not only to air power operations but also to every aspect of joint capability across defence, and in concert with our allies."

Britain's allies in the US played an important role in the Airseeker's development, with L-3 Mission Integration in Greenville, Texas, having converted three KC-135 air-air refuelling aircraft to RC-135 standard.

The US government also provided ground infrastructure, training of personnel and ground support systems.

The third aircraft arrived at RAF Waddington, in Lincolnshire, last year but is now deployable in support of national and coalition operations.